The present invention relates generally to catalysts. More specifically, the present invention relates to a heterogeneous catalyst based on silicon dioxide and a method of manufacturing same.
Heterogeneous catalysts are substances that increase the reaction speed of a chemical reaction. Heterogeneous catalysts proceed unmodified therefrom and are not present in the same phase as the substrates that are reacting.
Typically, the heterogeneous catalysts are in a solid state. Due to the solid phase of the heterogeneous catalysts, they are usually used with reactants that are in a liquid or gaseous phase. These solid heterogeneous catalysts are particularly easy to manipulate and can be easily separated from the reaction product and, therefore, are reusuable.
Heterogeneous reactions only occur at the phase boundary surfaces of the phases containing the reaction components. Given a solid catalyst and a liquid or gaseous substrates, therefore, the free surface of the catalyst that is available greatly influences the reaction speed. Accordingly, in order to further accelerate heterogeneously catalyzed reactions, it is necessary to create a catalyst having as large a surface area as possible.
Naturally occurring, porous materials, such as, for example, diatomaceous earth, bentonites, silica gel, pumice, aluminum silicates, zeolites, or aluminum oxide, have been used as catalysts for some fields of application. These materials are available as raw material, as may be derived, for example, from Roempps Chemie-Lexikon, edited by O.-A. Neumueller, 8th Edition, Stuttgart 1983, Volume 3, pages 2052-2058.
Although these initial materials can be utilized as catalysts for some fields of application, due to the acidic properties of the free Si-OH groups, materials containing silicon dioxide are utilized as acid/base catalysts. The acidic character of the catalyst material can be further varied by additives of suitable metal oxides, such as, for example, aluminum oxide, magnesium oxide, etc. In the reduction-oxidation catalysts, the easily reducible or unstable oxides of transition metals, such as for example, metals of the sub-groups 7a,8 and 1b of the periodic table (for example, iron, copper, nickel, etc.) form the catalytically active components.
Precipitation or impregnation methods are utilized for applying the catalytically active component to the carrier material. The catalytically active component, or a preliminary stage which can be converted thereinto, is fixed on the base member through a liquid phase (e.g., solution or melt). This can proceed both via a purely physical coating of the surface as well as via a chemical bonding, preferably with free Si-OH groups given silicon dioxide.
When the catalytically active component is fixed in the form of a preliminary stage, then an activation must also proceed. This can be a high-temperature step utilizing oxidizing or reducing conditions (e.g., O.sub.2 or H.sub.2 atmosphere).
Although these silicon dioxide containing catalyst systems are desirable, they suffer certain disadvantages. Silicon dioxide containing catalyst systems that are based on naturally occurring initial materials exhibit a relatively high degree of impurities. This makes the catalyst difficult, if not impossible, to use in many applications. A pure silicon dioxide is therefore usually preferred.
Silicon dioxide can be produced synthetically. However, synthetically produced silicon dioxide material which, for example, is produced from water glass or by flame hydrolysis of organic or inorganic compounds containing silicon exhibits high impurity and is too expensive to create when compared to natural material.
There is therefore a need for an improved heterogeneous catalyst that is based on silicon dioxide.